Multi-phase voltage-control oscillator

ABSTRACT

A multi-phase voltage-control oscillator including a first voltage-control oscillator circuit and a second voltage-control oscillator circuit is provided. The second voltage-control oscillator circuit and the first voltage-control oscillator circuit have a plurality of inductors, and the inductors in the second voltage-control oscillator circuit are respectively cross-coupled with the inductors in the first voltage-control oscillator circuit to generate a mutual inductance effect, so as to output a plurality of oscillating signals with different phases.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority benefit of anapplication Ser. No. 11/616,899, filed on Dec. 28, 2006, now allowed,which claims the priority benefit of Taiwan application serial no.95141131, filed on Nov. 7, 2006. The entirety of each of theabove-mentioned patent applications is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a multi-phase voltage-controloscillator (VCO) structure. More particularly, the present inventionrelates to a structure with inductors between which an inductance effectis generated.

2. Description of Related Art

Currently, with the progress of communication industry, integratedcircuits (IC) have been widely applied in wireless communication. Indesign, the ICs must meet several requirements such as low-voltageoperation and low power consumption. In a wireless communication system,after the antenna receives a radio frequency (RF) signal, a frequencydown converter constituted of an LNA, a VCO, and a mixer is used todown-convert the RF signal to an intermediate frequency signal. For thesignal to be transmitted, a frequency up converter constituted of a VCO,a mixer, and a high-power amplifier is used to up-convert theintermediate frequency signal to the RF signal, and then theup-converted signal is transmitted via the antenna. A RF microwavecircuit is located at the most front electrode of the entirecommunication system, and the characteristic of the RF microwave circuitdirectly affects the quality of communication, so the RF microwavecircuit is a crucial part.

The VCO plays an important role in modem communication systems. As thefrequency up- and down-conversion is mainly performed by the mixers atthe front electrode of the VCO and the transmitter/receiver, the noiseof the VCO will influence the noise level of the entire transceiver.Therefore, it is an important subject how to reduce the phase noise ofthe VCO.

Since the architecture adopted by the RF receiver is a distinct one, therequirements for the multi-phase signal become relatively strict. Aquadrature-phase signal, for example, can be generated by the followingmethods.

(1) Combination of VCO, polyphase-filter, and output buffers. In thismethod, four output buffers are required. If the four output buffers aredisposed between the filter and the VCO, the output buffer consumes alot of power. If the output buffer between the filter and VCO isremoved, the capacitance of a resonance cavity increases accordingly,resulting in large power consumption and the increase of the phasenoise. In addition, in order to obtain a good match of the filter, theintegration is the only choice. Therefore, the method of generating thequadrature-phase signal has a disadvantage of requiring large chip area.

(2) Frequency division. In this method, the required chip area is small.However, the structure must use a master-slave flip-flop which mustoperate at a frequency same as that of the VCO, i.e. twice of theoperating frequency. Therefore, this method has a problem of large powerconsumption.

(3) Two cross-coupled VCOs. The power consumption of this method is muchless than that of the above two forms. However, the method has adisadvantage that the use area is twice of a typical differential VCOarea.

The current quadrature-phase VCO (QVCO) mostly uses a so-called LC tankas a basic oscillator architecture and an additional MOS device togenerate a coupled signal. The method has lower phase noise, and theQVCO adopting the LC tank can be classified into two categories.

The first are those having the MOS device generating the coupled signalconnected in parallel with the MOS device generating a negativeresistance in the LC tank VCO architecture. Referring to FIG. 1, acircuit diagram of an LC tank QVCO is provided. The architecture ischaracterized in that the phase error of the quadrature-phase output isclosely relevant to the MOS device generating the coupled signal. Thebigger the MOS device generating the coupled signal is, the smaller thephase error of the quadrature-phase signal output by the QVCO is.However, the oversized coupled MOS device results in that the phasenoise get worse due to the extra coupled MOS device, and also incurs anadditional current consumption, which leads to a great increase of powerconsumption.

The second are those having the MOS device generating the coupled signalconnected in series with the MOS device generating the negativeresistance in the LC tank VCO architecture, in which there are twoplacement manners of the two MOS devices. Referring to FIG. 2, the MOSdevice generating the coupled signal can be placed at the source of theMOS device generating the negative resistance in the LC tank VCOarchitecture, which is called as the bottom-series QVCO. Referring toFIG. 3, the MOS device generating the coupled signal can also be placedat the drain of the MOS device generating the negative resistance in theLC tank VCO architecture, which is called as the top-series QVCO. Thetop-series QVCO has a better performance than the bottom-series QVCO.

Although the performance of the second series architecture is betterthan that of the parallel QVCO, as the stacked MOS devices are disposedin the path from the voltage supply electrode to ground electrode of theseries architecture, the circuit cannot operates under low voltagesupply.

Therefore, it is a problem to be solved how to operate the multi-phaseVCO under low voltage and maintain good performance.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to providing amulti-phase VCO capable of operating under low voltage and having goodperformance.

A VCO comprising a first VCO circuit and a second VCO circuit isprovided. The first VCO circuit and the second VCO circuit have aplurality of inductors, and the inductors in the first VCO circuit arerespectively coupled to the corresponding inductors in the second VCOcircuit to generate a mutual inductance effect, so as to output aplurality of oscillating signals.

According to the design concept of the multi-phase VCO of the presentinvention, the individually-operated inductors in the LC tank circuitare cross-coupled to generate an inductance, so as to replace the MOSdevice originally added to the circuit externally, such that themulti-phase VCO can operates under low supply voltage and has goodperformance.

In order to the make aforementioned and other features and advantages ofthe present invention comprehensible, preferred embodiments accompaniedwith figures are described in detail below.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and furtherexplanation of the invention as claimed is provided with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a circuit diagram of a conventional LC tank QVCO.

FIG. 2 is a circuit diagram of another conventional LC tank QVCO.

FIG. 3 is a circuit diagram of still another conventional LC tank QVCO.

FIG. 4 is a block diagram of a QVCO according to a preferred embodimentof the present invention.

FIG. 5 is a detailed circuit diagram of a multi-phase VCO according to afirst embodiment of the present invention.

FIG. 6 is a detailed circuit diagram of a multi-phase VCO according to asecond embodiment of the present invention.

FIG. 7 is a detailed circuit diagram of a multi-phase VCO according to athird embodiment of the present invention.

FIG. 8 is a detailed circuit diagram of a multi-phase VCO according to afourth embodiment of the present invention.

FIG. 9 is a detailed circuit diagram of a multi-phase VCO according to afifth embodiment of the present invention.

FIG. 10 is a detailed circuit diagram of the QVCO according to anotherembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

FIG. 4 is a block diagram of a multi-phase VCO according to a preferredembodiment of the present invention. The multi-phase VCO includestransformer feedback LC tank VCO circuits 10, 11 and a coupling unit 20.The transformer feedback LC tank VCO circuits 10, 11 are the main bodythat enables the multi-phase VCO to generate oscillating signals. Thecoupling unit 20 can make the originally individually-operatedtransformer feedback LC tank VCO circuits 10, 11 to generate thecoupling of signal, thereby generating a plurality of output signalswith different phases.

FIG. 5 is a detailed circuit diagram of a multi-phase VCO according to afirst embodiment of the present invention. Referring to FIG. 5, themulti-phase VCO provided by this embodiment includes VCO circuits 510and 530. The VCO circuit 510 includes transistors 512, 514, inductors516, 518, 520, 522, and varactors 524, 526. Moreover, the VCO circuit530 includes transistors 532, 534, inductors 536, 538, 540, 542, andvaractors 544, 546. Since the VCO circuits 510 and 530 are symmetric inthe circuit structure, in the following, only the VCO circuit 510 istaken as an example for illustration, and those of ordinary skill in theart can easily deduce the structure of the VCO circuit 530.

In the VCO circuit 510, the source electrodes of the transistors 512,514 are coupled with a voltage source VDD, and the drain electrodes arecoupled to the respective gate electrodes through the inductors 518 and516, respectively coupled to a control voltage Vtune through varactors524, 526, and are grounded through inductors 520, 522. In a preferredembodiment, transistors 512 and 514 are, but not limited to, NMOStransistors.

It should be noted that in VCO circuits 510, 530, inductors 516, 540,inductors 518, 542, inductors 520, 538, and inductors 522, 536 can becross-coupled to generate the mutual inductance effect, therebygenerating multiple phase oscillating signals. In addition, thetransistor 512 and transistor 514 are cross-coupled, and the transistor532 and the transistor 534 are cross-coupled, so as to generate thenegative resistance to compensate an equivalent parasitic impedancegenerated by the inductors 520, 522, 540, 542 and varactors 524, 526,544, 546 and to make the transformer feedback LC tank VCO circuits 10,11 keep oscillating.

In addition, inductors 516, 518, 536, 538 are used to obtain feedbacksignals, so as to improve the performance of the circuit. The varactoris constituted of MOS transistor or P-N diode, and the varactor iscoupled to the control voltage Vtune, so as to change the oscillatingfrequency of the LC tank.

FIG. 6 is a detailed circuit diagram of a multi-phase VCO according to asecond embodiment of the present invention. Referring to FIGS. 5 and 6,the multi-phase VCO provided by this embodiment includes VCO circuits610, 630. The VCO circuit 610 includes transistors 612, 614, inductors616, 618, 620, 622, and varactors 624, 626. In this embodiment,transistors 612, 614 are, for example, NMOS transistors, and the drainelectrodes thereof are respectively coupled to a voltage source VDDthrough inductors 620, 622, and are respectively coupled to a controlvoltage Vtune through varactors 624, 626, and the source electrodes aregrounded together. In addition, the gate electrodes of the transistors612, 614 are respectively coupled to the drain electrodes. Further, theback gate electrode of the transistor 612 can be grounded through theinductor 616. Similarly, the back gate electrode of the transistor 614is grounded through the inductor 618.

In another aspect, the VCO circuit 630 includes transistors 632, 634,inductors 636, 638, 640, 642, and varactors 644, 646. The circuitstructure of the VCO circuit 630 may be referred to the coupling mannerof the VCO circuit 610. The inductor 636 and the inductor 622 arecross-coupled, and the inductor 638 and the inductor 620 arecross-coupled, so as to generate oscillating signals of differentphases. In addition, the inductor 640 and the inductor 616 arecross-coupled, and the inductor 642 and the inductor 618 arecross-coupled, so as to generate a quadrature-phase oscillating signal.

FIG. 7 is a detailed circuit diagram of a multi-phase VCO according to athird embodiment of the present invention. Referring to FIG. 7, themulti-phase VCO provided by this embodiment includes VCO circuits 710and 730, and the circuit structures of the VCO circuits 710, 730 aresubstantially the same. Therefore, the VCO circuit 710 is taken as anexample for illustration as follows.

The VCO circuit 710 includes transistors 712 and 714, inductors 716,718, 720, 722, and varactors 724, 726. The transistors 712 and 714 canbe PMOS transistors, the source electrodes thereof are respectivelycoupled to the voltage source VDD through the inductors 716 and 718, andthe drain electrodes are coupled to the respective gate electrodes,coupled to the control voltage Vtune through the varactors 724, 726, andare grounded through the inductors 720, 722.

Similarly, the VCO circuit 730 also includes transistors 732, 734,inductors 736, 738, 740, 742, and varactors 744, 746, and the couplingmanner can be referred to the circuit structure of the VCO circuit 710.The inductors 736, 722, the inductors 738, 720, the inductors 716, 740,and the inductors 742, 718 are cross-coupled, so as to generate theoscillating signals of different phases.

FIG. 8 is a detailed circuit diagram of a multi-phase VCO according to afourth embodiment of the present invention. Referring to FIG. 8, the VCOprovided by this embodiment includes VCO circuits 810, 830 which areelectrically connected to each other and have mostly the same circuitstructures. The VCO 810 includes transistors 812, 814, inductors 816,818, 820, 822, varactors 824, 826, and a capacitor 858. The transistors812 and 814 can be PMOS transistors, and the source electrodes thereofare coupled to the voltage source VDD, and the drain electrodes arecoupled to the respective gate electrodes through the inductors 818,816, coupled to the control voltage Vtune through varactors 824 and 826,and are electrically connected to the VCO circuit 830 through inductors820, 822.

In addition, in the VCO circuit 810, the capacitor 858 is used to groundthe coupled node of the inductors 820, 822.

In another aspect, the VCO circuit 830 includes transistors 832, 834,inductors 836, 838, 840, 842, and varactors 844, 846. The transistors832, 834 are also NMOS transistors, the source electrodes are coupled tothe VCO circuit 810, and the drain electrodes are coupled to therespective gate electrodes through inductors 838 and 836, coupled to thecontrol voltage Vtune through varactors 844, 846, and are respectivelygrounded through inductors 840, 842.

Similarly, in this embodiment, the inductors 816, 840, the inductors818, 842, and the inductors 820, 838, and the inductors 822, 836 arecross-coupled, so as to generate a quadrature-phase oscillating signal.

The architecture disclosed in FIG. 8 makes use of the concept ofrepeated use of the current to greatly reduce the power consumption ofthe entire circuit. In FIG. 8, the mutual inductance relation betweeninductors can be used to replace the additionally added MOS devicegenerating the coupled signals in the original circuit, which can alsoachieve the coupling of signal and generate the quadrature-phase outputsignal. For the conventional circuit architecture repeatedly using thecurrent, the additionally added MOS devices require for a lot ofisolating capacitors and bias resistors to control the DC-bias level ofthe MOS devices, and this embodiment can solve the above problem.

FIG. 9 is a detailed circuit diagram of a multi-phase VCO according to afifth embodiment of the present invention. Referring to FIG. 9, the VCOprovided by this embodiment also includes VCO circuits 910 and 930. TheVCO circuit 910 includes transistors 912, 914, 916, inductors 918, 920,922, 924, and varactors 926, 928. The transistors 912, 914, 916 may bePMOS transistors, and the transistors 912 and 932 functions as thecurrent source to provide and limit the overall current consumption ofthe circuit. The source electrode of the transistor 912 is coupled tothe voltage source VDD, and the gate electrode is coupled to the biasvoltage Vbias, and the drain electrode is coupled to the sourceelectrodes of the transistors 914, 916. The drain electrodes of thetransistors 914, 916 are coupled to the respective gate electrodesthrough the inductors 920, 918, coupled to the control voltage Vtunethrough the varactors 926, 928, and are respectively grounded throughinductors 922, 924.

The VCO circuit 930 includes transistors 932, 934, 936, inductors 938,840, 942, 944, and varactors 946, 948. The circuit structure of VCOcircuit 930 can be referred to the VCO circuit 910. The inductors 918,942, inductors 920, 944, inductors 922, 940, and inductors 924, 938 arecross-coupled to generate oscillating signals of different phases.

FIG. 10 is a detailed circuit diagram of a multi-phase VCO according toa sixth embodiment of the present invention. Referring to FIG. 10, theVCO provided by this embodiment is a variation of the VCO provided bythe first embodiment (FIG. 5), and the difference lies in that the PMOStransistor is substituted by the NMOS transistor. Similarly, the VCOprovided by this embodiment also includes same VCO circuits 1010 and1030.

The VCO circuit 1010 includes transistors 1012, 1014, inductors 1016,1016, 1020, 1022, and varactors 1024, 1026. The transistors 1012, 1014can be PMOS transistors, the source electrodes are grounded, the drainelectrodes are coupled to the respective gate electrodes through theinductors 1018, 1016, coupled to the control voltage Vtune through thevaractors 1024, 1026, and are coupled to the voltage source VDD throughthe inductors 1020, 1022.

Similarly, the VCO circuit 1010 also includes transistors 1032, 1034,inductors 1036, 1038, 1040, 1042, and varactors 1044, 1046. Theinductors 1016, 1040, the inductors 1018, 1042, inductors 1020, 1038,and inductors 1022, 1036 are cross-coupled with each other, so as togenerate the oscillating signals of difference phase.

To sum up, the present invention adopts a new design concept, in whichthe individually-operated inductors in the LC tank circuit are utilizedto generate inductance, so as to replace the original MOS device addedon the circuit externally, such that the multi-phase VCO can operatesunder low supply voltage, the power consumption is effectively reduced,and the preferred performance is provided.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, the present invention covers modifications andvariations of this invention provided they fall within the scope of thefollowing claims and their equivalents.

1. A multi-phase voltage-control oscillator (VCO), comprising: a firstVCO circuit; and a second VCO circuit, wherein the second VCO circuitand the first VCO circuit have a plurality of inductors, and theinductors in the second VCO circuit are respectively cross-coupled withthe inductors in the first VCO circuit to generate a mutual inductanceeffect, so as to output a plurality of oscillating signals withdifferent phases.
 2. The multi-phase VCO as claimed in claim 1, whereinthe first VCO circuit comprises: a first NMOS transistor having a secondsource/drain electrode grounded; a second NMOS transistor having asecond source/drain electrode grounded, and a gate electrode coupled tothe first source/drain electrode of the first NMOS transistor; a firstinductor for grounding a back gate electrode of the first NMOStransistor; a second inductor for grounding a back gate electrode of thesecond NMOS transistor; a first varactor for coupling a firstsource/drain electrode of the first NMOS transistor to a controlvoltage; a second varactor for coupling a first source/drain electrodeof the second NMOS transistor to the control voltage; a third inductorfor coupling the first source/drain electrode of the first NMOStransistor to a voltage source; and a fourth inductor for coupling thefirst source/drain electrode of the second NMOS transistor to thevoltage source.
 3. The multi-phase VCO as claimed in claim 2, whereinthe second VCO circuit comprises: a third NMOS transistor having asecond source/drain electrode grounded; a fourth NMOS transistor havinga second source/drain electrode grounded and a gate electrode coupled toa first source/drain electrode of the third NMOS transistor; a fifthinductor for grounding a back gate electrode of the third NMOStransistor, and cross-coupled with the fourth inductor to generate afirst oscillating signal; a sixth inductor for grounding a back gateelectrode of the fourth NMOS transistor, and cross-coupled with thethird inductor to generate a second oscillating signal; a third varactorfor coupling a first source/drain electrode of the third NMOS transistorto the control voltage; a fourth varactor for coupling a firstsource/drain electrode of the fourth NMOS transistor to the controlvoltage; a seventh inductor for coupling the first source/drainelectrode of the third NMOS transistor to the voltage source, andcross-coupled with the first inductor to generate a third oscillatingsignal; and an eighth inductor for coupling a first source/drainelectrode of the fourth NMOS transistor to the voltage source, andcross-coupled with the second inductor to generate a fourth oscillatingsignal.
 4. The multi-phase VCO as claimed in claim 1, wherein the firstVCO circuit comprises: a first NMOS transistor; a second NMOS transistorhaving a gate electrode and a second source/drain electrode respectivelycoupled to a second source/drain electrode and a gate electrode of thefirst NMOS transistor; a first inductor for coupling a firstsource/drain electrode of the first NMOS transistor to a voltage source;a second inductor for coupling a first source/drain electrode of thesecond NMOS transistor to the voltage source; a first varactor forcoupling the second source/drain electrode of the first NMOS transistorto a control voltage; a second varactor for coupling the secondsource/drain electrode of the second NMOS transistor to the controlvoltage; a third inductor for grounding the second source/drainelectrode of the first NMOS transistor; and a fourth inductor forgrounding the second source/drain electrode of the second NMOStransistor.
 5. The multi-phase VCO as claimed in claim 4, wherein thefirst VCO circuit comprises: a third NMOS transistor; a fourth NMOStransistor having a gate electrode and a second source/drain electroderespectively coupled to a second source/drain electrode and a gateelectrode of the third NMOS transistor; a fifth inductor for coupling afirst source/drain electrode of the third NMOS transistor to the voltagesource, and cross-coupled with the fourth inductor to generate a firstoscillating signal; a sixth inductor for coupling a first source/drainelectrode of the fourth NMOS transistor to the voltage source, andcross-coupled with the third inductor to generate a second oscillatingsignal; a third varactor for coupling the second source/drain electrodeof the third NMOS transistor to the control voltage; a fourth varactorfor coupling the second source/drain electrode of the fourth NMOStransistor to the control voltage; a seventh inductor for grounding thesecond source/drain electrode of the third NMOS transistor, andcross-coupled with the first inductor to generate a third oscillatingsignal; and an eighth inductor for grounding the second source/drainelectrode of the fourth NMOS transistor, and cross-coupled with thesecond inductor to generate a fourth oscillating signal.
 6. Themulti-phase VCO as claimed in claim 1, wherein the first VCO circuitcomprises: a first NMOS transistor having a first source/drain electrodecoupled to a voltage source; a second NMOS transistor having a firstsource/drain electrode coupled to the voltage source; a first inductorfor coupling a gate electrode of the first NMOS transistor to a secondsource/drain electrode of the second NMOS transistor; a second inductor,for coupling a gate electrode of the second NMOS transistor to a secondsource/drain electrode of the first NMOS transistor; a first varactorfor coupling the second source/drain electrode of a first NMOStransistor to a control voltage; a second varactor for coupling a secondsource/drain electrode of the second NMOS transistor to the controlvoltage; a third inductor having an electrode coupled to the secondsource/drain electrode of the first NMOS transistor; a fourth inductorfor coupling the second source/drain electrodes of the first NMOStransistor and the second NMOS transistor through the third inductor;and a capacitor for grounding a coupled node of the third inductor andthe fourth inductor.
 7. The multi-phase VCO as claimed in claim 6,wherein the second VCO circuit comprises: a third NMOS transistor havinga first source/drain electrode coupled to the coupled node of the thirdinductor and the fourth inductor; a fourth NMOS transistor having afirst source/drain electrode coupled to the first source/drain electrodeof the third NMOS transistor; a fifth inductor for coupling a gateelectrode of the third NMOS transistor to a second source/drainelectrode of the fourth NMOS transistor, and cross-coupled with thefourth inductor to generate a first oscillating signal; a sixth inductorfor coupling a gate electrode of the fourth NMOS transistor to a secondsource/drain electrode of the third NMOS transistor, and cross-coupledwith the third inductor to generate a second oscillating signal; a thirdvaractor for coupling the second source/drain electrode of the thirdNMOS transistor to the control voltage; a fourth varactor for couplingthe second source/drain electrode of the fourth NMOS transistor to thecontrol voltage; a seventh inductor for grounding the secondsource/drain electrode of the third NMOS transistor, and cross-coupledwith the first inductor to generate a third oscillating signal; and aneighth inductor for grounding the second source/drain electrode of thefourth NMOS transistor, and cross-coupled with the second inductor togenerate a fourth oscillating signal.
 8. The multi-phase VCO as claimedin claim 1, wherein the first VCO circuit comprises: a first NMOStransistor having a first source/drain electrode coupled to a voltagesource and a gate electrode coupled with a bias voltage; a second NMOStransistor having a first source/drain electrode coupled to a secondsource/drain electrode of the first NMOS transistor; a third NMOStransistor having a first source/drain electrode coupled to the firstsource/drain electrode of the second NMOS transistor; a first inductorfor coupling a gate electrode of the second NMOS transistor to a secondsource/drain electrode of the third NMOS transistor; a second inductorfor coupling a gate electrode of the third NMOS transistor to a secondsource/drain electrode of the second NMOS transistor; a first varactorfor coupling the second source/drain electrode of the second NMOStransistor to a control voltage; a second varactor for coupling thesecond source/drain electrode of the third NMOS transistor to thecontrol voltage; a third inductor for grounding the second source/drainelectrode of the second NMOS transistor; and a fourth inductor forgrounding the second source/drain electrode of the third NMOStransistor.
 9. The multi-phase VCO as claimed in claim 8, wherein thesecond VCO circuit comprises: a fourth NMOS transistor having a firstsource/drain electrode coupled to the voltage source and a gateelectrode coupled with the bias voltage; a fifth NMOS transistor havinga first source/drain electrode coupled to a second source/drainelectrode of the fourth NMOS transistor; a sixth NMOS transistor havinga first source/drain electrode coupled to the first source/drainelectrode of the fifth transistor; a fifth inductor for coupling a gateelectrode of the fifth NMOS transistor to a second source/drainelectrode of the sixth NMOS transistor, and cross-coupled with thefourth inductor to generate a first oscillating signal; a sixth inductorfor coupling a gate electrode of the sixth NMOS transistor to a secondsource/drain electrode of the fifth NMOS transistor, and cross-coupledwith the third inductor to generate a second oscillating signal; a thirdvaractor for coupling a second source/drain electrode of the fifth NMOStransistor to the control voltage; a fourth varactor for coupling asecond source/drain electrode of the sixth NMOS transistor to thecontrol voltage; a seventh inductor for grounding the secondsource/drain electrode of the fifth NMOS transistor, and cross-coupledwith the first inductor to generate a third oscillating signal; and aneighth inductor for grounding the second source/drain electrode of thesixth NMOS transistor to ground, and cross-coupled with the secondinductor to generate a fourth oscillating signal.
 10. The multi-phaseVCO as claimed in claim 1, wherein the first VCO circuit comprises: afirst PMOS transistor having a second source/drain electrode grounded; asecond PMOS transistor having a second source/drain electrode grounded;a first inductor for coupling a gate electrode of the first PMOS to afirst source/drain electrode of the second PMOS transistor; a secondinductor for coupling a gate electrode of the second PMOS transistor toa first source/drain electrode of the first PMOS transistor; a firstvaractor for coupling the first source/drain electrode of the first PMOStransistor to a control voltage; a second varactor for coupling thefirst source/drain electrode of the second PMOS transistor to thecontrol voltage; a third inductor for coupling the first source/drainelectrode of the first PMOS transistor to a voltage source; and a fourthinductor for coupling the first source/drain electrode of the secondPMOS transistor to the voltage source.
 11. The multi-phase VCO asclaimed in claim 10, wherein the second VCO circuit comprises: a thirdPMOS transistor having a second source/drain electrode grounded; afourth PMOS transistor having a second source/drain electrode grounded;a fifth inductor for coupling a gate electrode of the third PMOStransistor to a first source/drain electrode of the fourth PMOStransistor, and cross-coupled with the fourth inductor to generate afirst oscillating signal; a sixth inductor for coupling a gate electrodeof the fourth PMOS transistor to a first source/drain electrode of thethird PMOS transistor, and cross-coupled with the third inductor togenerate a second oscillating signal; a third varactor for coupling thefirst source/drain electrode of the third PMOS transistor to the controlvoltage; a fourth varactor for coupling the first source/drain electrodeof the fourth PMOS transistor to the control voltage; a seventh inductorfor coupling the first source/drain electrode of the third PMOStransistor to the voltage source, and cross-coupled with the firstinductorto generate a third oscillating signal; and an eighth inductorfor coupling the first source/drain electrode of the fourth PMOStransistor to the voltage source, and cross-coupled with the secondinductor to generate a fourth oscillating signal.